There is provided a method for calibrating an antenna. More precisely there is provided a method for calibrating an antenna where at least one switch is spanning sections of a radiating antenna element, and where a controller is designed to selectively close the switch, thus effectively short-circuiting the actual section of the radiating antenna element. The invention also includes a device for calibrating an antenna.
Resonant antennas having one or more radiating elements may be tuned to work over a broad range of frequencies by short-circuiting at least a section of the radiating antenna element. The short-circuiting may be performed by the use of a mechanically operated or semiconductor based switch.
It is well known technique to use so called PIN diodes for this purpose. The PIN diode is chosen due to the fact that it has a poor reverse recovery time and will stay open at high frequencies.
In U.S. Pat. No. 4,924,238, Ploussios outlines a resonant helical electronically tuned antenna. The antenna is tuned by a series of oppositely poled pairs of diodes that are connected at spaced points to the radiating coils of the antenna. When the diodes are biased to be conductive, a section of the radiating helix is short-circuited. Bias voltages to control the diodes are provided by a bias voltage generator. See FIG. 1.
The bias voltage generator may be programmed to produce a voltage for a specific pair of diodes when the antenna is transmitting or receiving signals at a specific frequency range. The control of the biasing voltage generator may include a factory made table that states which diodes to bias at which voltage range. The factory calibration process may take some time.
Experience has shown that great production and component precision is necessary if antennas of this kind are to be calibrated at the factory. Further the topography and local conditions at the position of installation are known to greatly influence the way the antenna behaves. It is also influenced by the ambient temperature and ageing.